Control devices which analyze the data from sensors and forward them to a display are used for reversing aids in a vehicle. Cables separate from the rest of the electrical system are incorporated into the vehicle for data transmission from the control device to the display, which must be placed at a location accessible to the driver. This means additional outlay in terms of cost and working time when a vehicle is manufactured. In a situation in which a reversing aid is to be retrofitted into a vehicle, cable routing is again the actual problem, as known from the article xe2x80x9cPilot projectxe2x80x9d [Pilotprojekt], Gute Fahrt 10/91, pp. 50-53. The outlay is particularly great if a tractor vehicle having a trailer or semitrailer is to be equipped with reversing aids. The coupling of the trailer and its electrical connection via a special cable, e.g. a helical cable with a waterproof connector, particularly increases the outlay.
The method according to the present invention for data transmission, has, in contrast, the advantage of resorting to existing cable runs in the vehicle. Especially in the case of possible retrofitting of a vehicle with a reversing aid, it is much simpler to use the already existing cable runs as data lines and supply lines for the control devices.
The simplest possibility for data transmission is via the electrical supply line to which the control devices are also connected. It is particularly advantageous in this context if the lines for loads such as, for example, the reversing lights are used as the electrical supply line. This has the advantage that the control device is supplied with voltage only in a situation in which the vehicle is moving backward. The switch already present in the vehicle for reverse gear then serves to switch the control device on and off.
Data transmission can also be accomplished via a further electrical supply line which does not supply power to the control devices themselves.
In a further embodiment, the data transmission line selected is an electrical supply line, e.g. the two-wire line, of a directional indicator. Advantageously, the supply line for the reversing light is also selected for data transmission via a two-wire line. For data transmission via the directional indicator line, the loads, i.e. the directional indicators, need not be in operation. Another advantageous embodiment transmits the data symmetrically over both directional indicator lines that are present.
For use in a vehicle that has a tractor vehicle and a trailer, it is advantageous if only a first control device is authorized to forward data. The further control devices that are connected interrupt their data transmission if they detect a further control device.
Advantageous monitoring of the second control device to determine whether a first control device is connected is accomplished, for example, by eavesdropping on the signals of the first device in the data stream. For this purpose, the second control devices are equipped with a time delay. In this delay time, they eavesdrop on data that the first control device is sending immediately.
It is furthermore possible to detect a first control device by measuring the current of the electrical load.
All known transmission techniques can be used as the modulation and coding method, but frequency shift keying is preferably used.